The present invention relates to a bolt screwing into a female screw stripping the thick coating film or the spatter from said female screw, more particularly a bolt consisting of a column part and a polygonal pillar part extending from an end of said column part wherein a complete thread is formed on said column part and a screw blade continuing to said complete thread is formed on said polygonal pillar part. Further, the present invention relates to a manufacturing method of said bolt comprising; molding a green bolt body having a column part and a polygonal pillar part by forging work, and forming a complete thread on said column part and a screw blade on said polygonal pillar part at the same time by the thread rolling process.
A thick coating film for anti-corrosion is formed on the female screw by cation coating. Hitherto, it has been a problem that when the bolt is screwed into said female screw, said thick coating film may clog in the thread of the bolt resulting in the seizure of the bolt to the female screw.
To prevent the seizure of the bolt when the bolt is screwed into the female screw, a bolt having a notch at its end shown in FIG. 14 has been provided. Said bolt (11) consists of a head (12) and a column part (13) on which a thread is formed and said notch (15) is formed at the end part (13A) of said column part (13), wherein said column part (13) and said end part (13A) have the same diameter D4 respectively. When said bolt (11) is screwed into the tapped hole of the female screw (14) to which the thick film by the cationic coating, the spatter and the like adhere, said thick coating film, the spatter and the like may be stripped and removed by said notch (15) of said bolt (11).
Nevertheless, since said notch has an asymmetric trigonal shape spreading its width toward the top end of the bolt, so that a lot of part of the thread (14A) formed on the top end part of the bolt is incomplete, said bolt having a notch has a fault that the encroaching and the seizure between the bolt and the female screw are caused by an oblique screwing of the bolt into the female screw when the bolt and the female screw are fastened together.
Further, it is feared that the producting cost becomes high since 2 processes, forming a thread on the green bolt body by the thread rolling process and forming the notch on the bolt by the cutting work process, are necessary to manufacture said bolt having a notch.
The object of the present invention is to prevent the encroaching and the seizure between the bolt and the female screw caused by an oblique screwing of the bolt into the female screw when the bolt and the female screw are fastened together.
Another object of the present invention is to manufacture the bolt in a low cost, by using said bolt, the encroaching and the seizure between the bolt and the female screw caused by a oblique screwing of the bolt into the female screw will certainly be prevented.
Said objects may be attained by a bolt consisting of a column part and a polygonal pillar part extending from an end of said column part wherein a complete thread is formed on said column part and a screw blade continuing to said complete thread is formed on said polygonal pillar part.
Preferably, said complete thread of said column part and said screw blade of said polygonal pillar part have the same pitch respectively and said screw blade consists of a plural number of blade units spirally continuing respectively, each blade unit consists of a crest part situating at the corner of said polygonal pillar part and a flank part situating at the side of said polygonal pillar part.
Further, it is preferable that the full diameter of said screw blade of said polygonal pillar part is set to be equal to, or a little larger or a little smaller than, the full diameter of said complete thread of said column part.
Still further, it is preferable that a guide boss having a smaller diameter than said column part is extended from the end of said polygonal pillar part and a spiral guide groove is formed on said guide boss, said spiral guide groove has the same pitch as the root of said complete thread of said column part and said guide groove is partially deeper than the root of said complete thread.
The end of said guide groove has preferably a discharging groove to discharge the scobe extending from the flank part of said screw blade along the axis of said bolt crossing said guide groove of said guide boss and preferably said polygonal pillar part is trigonal to dodecagonal.
Further, said objects may be attained by a manufacturing method of a bolt comprising; molding a green bolt body having a column part and a polygonal pillar part by forging work, and forming a complete thread on said column part and a screw blade on said polygonal pillar part at the same time by the thread rolling process.
Preferably, a guide boss having a smaller diameter than said column part is formed at the end of said polygonal pillar part by the drawing process at the same time when said complete thread and said screw blade are formed by the thread rolling process and a spiral guide groove is formed on said guide boss by the thread rolling process at the same time when said guide boss is formed by the drawing process so that said spiral guide groove has the same pitch as the root of said complete thread of said column part and said guide groove is partially deeper than the root of said complete thread.
Since a complete thread (5A) screwing into a female screw (7) is formed on the column part in said bolt of the present invention, and a screw blade (5B) is formed on the polygonal pillar part (4), when said bolt (1) is screwed into a female screw (7), first said screw blade (5B) scrapes the thick film by the cationic coating, the spatter and the like adhering to said female screw (7) and then the complete thread (5A) of the column part of said bolt (1) engages said female screw (7)